1. Field of the Invention
The present invention concerns a method of acquiring ultrasound images, in particular in medical applications. The object of the invention is to enable ultrasound images obtained at different times in the life of a patient to be compared to determine whether pathological conditions indicated by a first image have developed with time.
2. Description of the Prior Art
The general principle of acquiring ultrasound images is familiar. It uses ultrasound equipment comprising a probe incorporating piezo-electric transducers. The equipment also includes control circuitry connected to the transducers which emit and receive ultrasound waves. To obtain an image of a part of the body of a patient the probe is placed against the skin of the patient near the part concerned. The ultrasound waves emitted by the probe are reflected by the tissues of the part of the body being examined and are received by the probe. This converts the ultrasound waves into electrical signals which are then processed. The main purpose of this signal processing is to produce an image of a cross-section of the body in the area concerned (a tomograph).
The cross-section to be imaged is scanned by the ultrasound waves. Two-dimensional scanning of the cross-section plane by the ultrasound waves is obtained for a given direction of emission by establishing the relationship between the signals received and the time at which they are received. Given the propagation speed of ultrasound waves in human tissue (which is in the order of 1,500 meters per second, given the high water content of human tissue), the signals reflected from deeper areas of the body reach the probe later. The signals from deeper parts of the body are usually attenuated. This attenuation can be compensated by applying amplification with a gain that varies with time.
To scan in the other direction the direction of emission is changed. There are various scanning modes. In lateral scanning the direction of emission is moved parallel to itself. In angular scanning the emission directions cover a portion of a circle (a fan shape).
The images produced by a probe can be displayed on a visual display unit in real time by electronically scanning the emission direction in synchronism or in some other corresponding relationship with the vertical scanning of the monitor. To this end the monitor video signal is conditioned by the amplitude of the detected, demodulated and filtered ultrasound signal.
Ultrasound equipment usually incorporates means for storing images. Rather than using an analog representation of the received ultrasound signals, the signals are digitized and stored in digital form. It is then possible, using an image memory and a display monitor, to display digitized images rather than the analog images themselves. Modern equipment uses methods of digitizing the analog signals produced by the probe so quickly that even with a digital image store real time display is possible. The display monitor shows the image contained in the image store. This is continually updated with new images produced by further scans.
The images produced enable a practitioner to detect the presence of particular pathologies. In the vessels of the neck, for example, the practitioner is looking for lesions or the appearance of atheroma plaques whose brightness in the image depends on how far advanced is the process of lipid-fibrosis-calcification. This is because their density increases their reflection coefficient. By reducing the flexibility of the blood vessels, the atheroma plaques slow the flow of blood and can eventually cause lesions to the brain through inadequate blood supply. It is therefore important to monitor this development. An image is therefore obtained of a region likely to contain or already containing such atheroma plaques. Later, for example a few months later, an equivalent image is obtained of the same part of the body. The two images are then compared in order to evaluate the degree to which the pathology in question has developed.
This comparison is difficult. It is affected by variations in image acquisition conditions. One attempt to solve this problem was described in 1983 in Radiology volume 148, no 2, pages 533-537 by David H. BLANDENHORN et al, in an article entitled "Common carotid artery contours reconstructed in three dimensions from parallel ultrasonic images". In the technique described, the probe is fastened to a motor-driven boom which is adapted to move the probe forward by calibrated amounts in order to obtain parallel sections. This produces a three-dimensional reconstruction of the relevant part of the patient's body. A similar process could be conducted on the same patient six months later. The 3D reconstructions could then be examined for corresponding tomographs in order to evaluate any development that may have taken place. Apart from the complexity of the equipment used, in particular to achieve calibrated movement of the probe, this technique could not yield conclusive results as the images depend on the power emitted each time. The images must therefore be calibrated. The theory of acoustical absorption by tissue does not provide any means of normalizing the power output of the emitters. The experiment is adversely affected by drift in the power output of the equipment. The brilliance is influenced by the power output. Also, the non-linear nature of the absorption phenomena means that normalization is not possible.
In another publication, "Evaluation of a scoring system for extracranial carotid atherosclerosis extent with B-mode ultrasound" by John R. CROUSE et al published in STROKE--volume 17 no 2 March-April 1986, pages 270 through 275, measurements of the severity, in other words the thickness of the largest atheroma plaque detected in a patient were carried out at different times for a sample of patients. This study concluded that the correlation between the first and second sets of measurements was low, given that attention was focused on the most severe lesion. However, there was better correlation between so-called "extension" measurements, relating to the sum of the axial thicknesses of all the local obstructions. Although the conclusions of this publication provide useful statistical elements, their teaching in response of individual patients is simply stated: the difficulty of comparing images acquired at one tim with images acquired at another time means that evaluating the development of a pathology is doubtful.
X-ray angiography and other techniques use image recalibration techniques in which one of two images is processed and then subtracted from the other. Distortion is corrected before the image is superimposed on the other image. This technique requires powerful computer systems (it uses very large amounts of memory) and cannot be transported to the ultrasound domain because the orientation of the ultrasound probe relative to the body of the patient is not maintained as accurately as is the orientation of an X-ray tube relative to the body of the patient in an X-ray angiography application.
The object of the invention is to remove these drawbacks by proposing an entirely different method which is directly accessible to all practitioners using existing ultrasound equipment. The invention produces not only statistical results that can be applied to a population of patients, but also results relating to each individual patient. The invention, differing in this respect from prior art practise, is based on the observation that practitioners themselves, with some experience, can perceive in real time the structures that they wish to demonstrate. It is considered, for example, that after carrying out ultrasound examinations of some 300 to 400 different patients, the practitioner acquires considerable skill in manipulating the probe in such a way as to show on the display what he wishes to see, in particular to acquire a first image, the lesions and the atheroma plaques in the carotid arteries of the neck.
Rather than apply distortion correcting processing to images acquired subsequently by these practitioners, the idea is to impose an additional constraint on practitioners with regard to acquiring the second images. This additional constraint requires them to place the probe in exactly the same position to obtain the second image as when the previous image was acquired. To embody this constraint in concrete terms, there is produced as a background to the real time display screen the contours of the tissues examined when the first image was acquired.
This requires the practitioner to display an image representing the lesion and to refine the representation of the lesion by manipulating the probe (by moving his hand) so that the second image is rendered as accurately coincident as possible with the background image. This process might be described as placing the second image in the first image. The background image may be highlighted (displayed with increased brightness). Practical experience by the inventors has shown that around 50 additional examinations are sufficient to obtain sufficient skill for a second image to be regarded as having been acquired with substantially the same conditions of probe inclination and incidence angle as the first image.
To solve the problem of power variation, part of the displayed image includes information on tissue density. This information is obtained by measuring the average brightness of the picture elements (pixels) contained in a geometrical figure (a circle) in the first image and the same figure at the same place in the new image. By adjusting a gain control, the practitioner operating the equipment can then adjust the power and/or the waveform of the ultrasound pulses until the measurements are compatible. When this has been done, the change in the brightness of the atheroma plaques can be determined to deduce whether or not it has become more calcified since the last examination.
This visual validation is regarded as subjective in the sense that it requires human evaluation of the measured resemblance. It is related to the discernment of the practitioner and is not the result of objective processing applied by a machine using an automatic process. Nevertheless, this subjective visual validation has advantages over previous practise. In particular, the second image can be acquired by a different practitioner from the practitioner who obtained the first image. Also, the fact that a contour from the first image is shown produces better results than would be obtained by attempting to present all of the first image.
In some known systems the display screen is divided into two parts in real time, the lefthand part, for example, showing the first image and the righthand part showing the new image being acquired. In such cases, given the existence of a lesion or other pathology in the first image, the practitioner works instinctively towards a second image in which the lesion appears larger, the same size or smaller, depending on whether the practitioner is unconsciously motivated to show an aggravation, a stagnation or a regression of the pathology. In this case the subjective element is too strong and the measurements are unreliable.
However, by showing only the contour of the first image, and optionally the density information, the practitioner is obliged to acquire an image which agrees as closely as possible with the older image without knowing what is expected on subsequently comparing the second image with the complete first image (and not just its contours). It is only then that the pathological areas in the new image are compared with the first image. The approach is different in the sense that the importance of the previously measured pathology is not revealed to the practitioner in real time, but later as an objective result of comparison. In effect, as the pathology is not present in the contour image displayed, it cannot influence the practitioner.
The invention therefore secures morphological and densitometric reproducibility of images based on the mental integration aptitudes of the practitioner manipulating the probe. This additional aptitude is obtained at the cost of only some 50 experiments, as has been established. The required dexterity is then obtained. Experience shows that the practitioner chooses the contours in the first image more and more quickly with practise: this means even with a small number of points. It has been found that reproducibility is not total, however. On the other hand, it is clearly better than what has been achieved previously. For example, it has been possible to perceive growth of lesions in the order of 5%, which was not possible with earlier techniques.
In one embodiment of the invention, the fact that modern ultrasound equipment incorporates an image store and means for storing digitized images is exploited. As the practitioner manipulates the probe, in the proximity of the final orientation and inclination that is required, the various images obtained are stored in real time. Subsequently, off-line, the best image can be chosen from the series of images, by which is meant the image that fits best by superposing the contours shown on the screen and relating to the first image.
It has been found that this technique is of particular benefit when studying cyclic phenomena. It is therefore particularly interesting in studying the phenomenon of blood flow in the arteries, in itself another limitation of older techniques.